On Thu, Jul 01, 2021 at 09:48:33AM +0100, Dr. David Alan Gilbert wrote:

* Theodore Ts'o (tytso@mit.edu) wrote:

quoted

On Wed, Jun 30, 2021 at 04:01:42PM +0100, Dr. David Alan Gilbert wrote:

quoted

Even if you fix symlinks, I don't think it fixes device nodes or
anything else where the permissions bitmap isn't purely used as the
permissions on the inode.

I think we're making a mountain out of a molehill.  Again, very few
people are using quota these days.  And if you give someone write
access to a 8TB disk, do you really care if they can "steal" 32k worth
of space (which is the maximum size of an xattr, enforced by the VFS).

OK, but what about character mode devices?  First of all, most users
don't have access to huge number of devices, but let's assume
something absurd.  Let's say that a user has write access to *1024*
devices.  (My /dev has 233 character mode devices, and I have write
access to well under a dozen.)

An 8TB disk costs about $200.  So how much of the "stolen" quota space
are we talking about, assuming the user has access to 1024 devices,
and the file system actually supports a 32k xattr.

    32k * 1024 * $200 / 8TB / (1024*1024*1024) = $0.000763 = 0.0763 cents

A 2TB SSD is less around $180, so even if we calculate the prices
based on SSD space, we're still talking about a quarter of a penny.

Why are we worrying about this?

I'm not worrying about storage cost, but we would need to define what
the rules are on who can write and change a user.* xattr on a device
node.  It doesn't feel sane to make it anyone who can write to the
device; then everyone can start leaving droppings on /dev/null.

Looks like tmpfs/devtmpfs might not support setting user.* xattrs. So
devices nodes there should not be a problem.

# touch /dev/foo.txt
# setfattr -n "user.foo" -v "bar" /dev/foo.txt
setfattr: /dev/foo.txt: Operation not supported

Vivek

The other evilness I can imagine, is if there's a 32k limit on xattrs on
a node, an evil user could write almost 32k of junk to the node
and then break the next login that tries to add an acl or breaks the
next relabel.

Dave

quoted

						- Ted

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK